Thermal ink jet printers are well known in the art and are illustrated in U.S. Pat. Nos. 4,490,728 and 4,313,684. The thermal ink jet printhead has an array of precisely formed nozzles, each having a chamber which receives ink from an ink reservoir. Each chamber has a thin-film resistor, known as a firing resistor, located opposite the nozzle so ink can collect between the nozzle and the firing resistor. When printing pulses heat the firing resistor, a small portion of the ink directly adjacent to the firing resistor vaporizes. The rapidly expanding ink vapor displaces ink from a nozzle causing drop ejection. The ejected drops collect on a print medium to form printed characters and images.
Printhead temperature fluctuations have prevented the realization of the full potential of thermal ink jet printers because these fluctuations produce variations in the size of the ejected drops which result in degraded print quality. The size of ejected drops varies with printhead temperature because two properties that control the size of the drops vary with printhead temperature: the viscosity of the ink and the amount of ink vaporized by a firing resistor when driven with a printing pulse. Printhead temperature fluctuations commonly occur during printer startup, during changes in ambient temperature, and when the printer output varies. For example, temperature fluctuations occur when the printer output changes from normal print to "black-out" print (i.e., where the printer covers the page with dots).
When printing text in black and white, the darkness of the print varies with printhead temperature because the darkness depends on the size of the ejected drops. When printing gray-scale images, the contrast of the image varies with printhead temperature because the contrast depends on the size of the ejected drops.
When printing color images, the printed color varies with printhead temperature because the printed color depends on the size of all the primary color drops that create the printed color. If the printhead temperature varies from one primary color nozzle to another, the size of drops ejected from one primary color nozzle will differ from the size of drops ejected from another primary color nozzle. The resulting printed color will differ from the intended color. When all the nozzles of the printhead have the same temperature but the printhead temperature increases or decreases as the page is printed, the colors at the top of the page will differ from the colors at the bottom of the page. To print text, graphics, or images of the highest quality, the printhead temperature must remain constant.
Thermal printers are well known in the art. The printheads have an array of heating elements that either heat thermal paper to produce a dot on the thermal paper or heat a ribbon (which can have bands of primary color inks as well as black ink) to transfer a dot to the page. In either case, fluctuations in the printhead temperature produce fluctuations in the size of the printed dot which affect the darkness of the print when printing in black and white, the gray-tone when printing in gray scale, and the resulting printed color when printing in color.